Operators of electric utility mobile work platforms of the cherry picker type or "man baskets" are often required to be on the platform while it is positioned adjacent to high voltage electrical transmission lines to perform installation, repair or maintenance tasks. Such operation requires the platform and its supporting boom be essentially electrically isolated from ground to minimize leakage currents which otherwise could establish a hazardous electrical voltage on the platform in the event the boom, operator or "man baskets" contacted or came into close proximity to a transmission line.
A typical system for controlling operation of the boom includes hydraulic cylinders mounted on the base unit to position the boom. Control of the cylinders is typically effected using manually operated control valves mounted on the platform basket connected to the actuator cylinders by means of flexible hoses extending between the basket and the base unit. This system requires several expensive and bulky hoses to be connected to the basket. These hoses must be kept free of conductive deposits and moisture to maintain the high value of outer surface resistivity necessary to prevent establishing a hazardous electrical voltage on the platform if the boom or basket were to contact high voltage wiring.
The use of a hydraulic system controlled from the "man basket" has many disadvantages in utility service. The system is bulky, costly and requires considerable maintenance to avoid leakage of hydraulic fluid and the resultant possibility of conductive deposits on connecting hoses. Additionally, hydraulic systems may not provide adequate control or instrumentation capability for more demanding applications.
Electrical control of mobile platforms, although offering advantages of lower cost, greater control flexibility and less bulk, is undesirable if conventional electric cables are used. They are not adequately isolated from ground and have increased risk of hazardous leakage current to ground in the event of contact with the high voltage transmission lines.
Optical conductors can transmit control and instrumentation information but such conductors are fragile and may require additional provisions to increase their tensile strength and environmental resistance to be suitable for industrial use, particularly in mobile applications. Typically, this can be accomplished by placing the optical conductor in a buffer tube. For further information concerning the structure and operation of cables, including optical conductors, reference may be made to commonly assigned U.S. Pat. Nos. 4,695,127 and 4,552,432.